Showing papers in "Astrophysics and Space Science in 1996"

Interstellar extinction from infrared to X-rays: An overview

Abstract: Interstellar extinction is presented in a unified picture from the infrared (λ=104 nm or 10 μm) to the X-rays (E=1.25 nm or 10 keV), as a cross-section normalized per hydrogen atom. Analytical expressions for the cross-section covering the whole range are cited or given in the Appendix for computational purposes.

Chemically Peculiar Hot Stars

Abstract: The upper main sequence is home to a diverse family of chemically peculiar stars, including λ Boo, Am-Fm, Bp-Ap, HgMn, He-weak, and He-rich varieties. This paper presents an informal review of the physical properties of these objects, including their location in the H-R diagram, frequency of incidence, rotation, binarity, magnetic fields, and variability. Part of the discussion is devoted to describing the bizarre surface compositions encountered in chemically peculiar stars, with an emphasis on insights provided by a generation of ultraviolet observations obtained with the IUE satellite. The paper concludes with an overview of the radiative diffusion mechanism which has been proposed as an explanation of the chemically peculiar phenomenon.

Non-linear stability zones around triangular equilibria in the plane circular restricted three-body problem with oblateness

Abstract: Non-linear stability zones of the triangular Lagrangian points are computed numerically in the case of oblate larger primary in the plane circular restricted three-body problem. It is found that oblateness has a noticeable effect and this is identified to be related to the resonant cases and the associated curves in the mass parameter μ versus oblateness coefficientA 1 parameter space.

Foil X-ray mirrors

Abstract: Nested thin foil reflectors have made possible light weight, inexpensive and fast grazing incidence X-ray mirrors for astronomical spectroscopy over a broad band. These mirrors were developed at Goddard for the US Shuttle program and were flown on NASA's shuttleborne Astro-l mission in December 1990. Presently, the Japan/US collaborative spectroscopic mission ASCA, nearing its third year of successful operation in earth orbit, carries, four such mirrors, weighing less than 40 kg and giving total effective areas of ∼ 1200 and 420 cm2 at l and 8 keV respectively. The ∼ 420 kg observatory is the best possible example of how conical foil mirrors opened areas of research that could not have been otherwise addressed with available resources. In this paper, we will briefly review the development and performance of our first generation foil mirrors. We will also describe progress toward improving their imaging capability to prime them for use in future instruments. Such a goal is highly desirable, if not necessary for this mirror technology to remain competitive for future applications.

Single layer atmospheric turbulence demonstrated by adaptive optics observations

Abstract: Carried with an astronomical adaptive optics (AO) system, this work reports the observational evidences of wave-front deformations dominated by sources of perturbation acting as phase slabs moving at a constant speed in front of the telescope aperture. Consequences for improved adaptive optics compensation are suggested.

Electrostatic solitons in multispecies electron-positron plasmas

Abstract: Acoustic solitons are investigated in electron-positron plasmas containing equal hot and cool components of both species. The hot components are isothermal Boltzmann distributed, the cool constituents are modelled by adiabatic fluids. The equations are integrated exactly in terms of a Sagdeev potential. Solitons are shown to be possible, but no double layers, due to the symmetry in the model. Bearing in mind the constraints imposed by the Boltzmann assumption, small amplitude solitons only are found. Such findings are relevant for different kinds of astrophysical plasmas, as well as for other types of similar acoustic solitons.

An essay on terminology, myths,- and known facts: Solar transient-flare-CME-driver gas-piston-BDE-magnetic cloud-shock wave-geomagnetic storm

Abstract: In the field of solar-terrestrial relations a clear and unique terminology is needed in order to abolish and avoid unnecessary confusion between the scientists from several involved disciplines. For example, the widely used abbreviationCME (for coronal mass ejection) has turned out to be somewhat misleading. Early on it had been known that other than coronal material is often involved in such events. The discoverers observed transient events of mass ejections from the sun, which could be observed in the corona owing to the newly available coronagraphs. This article is meant to clarify the terminology, with emphasis on giving credit to the original discoverers and the terms they introduced. With this aim in mind I suggest some minor modifications of the terminology.

Plane-Symmetric Mesonic Viscous Fluid Cosmological Model

Abstract: Exact nonstatic solutions to Einstein field equations are obtained for the plane symmetric spacetime filled with viscous perfect fluid in the presence of attractive scalar fields. Some physical and geometrical properties of the model are studied. The solutions characterize strong interaction of elementary particles.

Instabilities in astrophysical jets

Abstract: The observed properties of astrophysical jets are reviewed, and the techniques used to estimate the parameters of the underlying beams are described. This information is then used in a theoretical treatement of the Kelvin-Helmholtz instability of the flows, and the relevance of this instability to the persistence of the observed structures is emphasised.

A Bimodal Model of the Solar Wind Speed

Abstract: Until the ULYSSES spacecraft reached high latitude, the only means for measuring the solar wind velocity in the polar regions was from radio scattering observations (IPS), and these remain the only way to measure the velocity near the sun However, IPS, like many remote sensing observations, is a “line-of-sight” integrated measurement This integration is particularly troublesome when the line-of-sight passes through a fast stream but that stream does not occupy the entire scattering region Observations from the HELIOS spacecraft have shown that the solar wind has a bimodal character which becomes more pronounced near the sun Recent observations from ULYSSES have confirmed that this structure is clear at high latitudes even at relatively large solar distances We have developed a method of separating the fast and slow contributions to an IPS observation which takes advantage of this bimodal structure In this paper I will describe the technique and its application to IPS observations made using the receiving antennas of the EISCAT incoherent backscatter radar observatory in northern Scandinavia

The Darwin Project

Abstract: Angel and co-workers have proposed to detect exoplanets around nearby stars in the infrared (6–17 μm) and to analyze their spectra, searching for H2O, CO2, CH4, NH3, and O3 spectral features. The presence or absence of CO2 would indicate either a strong similarity or difference with Solar telluric planet atmospheres. Water would indicate a habitable planet, and O3 would reveal significant photosynthesis activity, due to the presence of carbon chemistry based life. Like these authors, we suggest an infrared nulling interferometer pointing to the star and working as a coronograph. Our main contribution is to propose an observatory made of four to five 1-meter class telescopes observing from about 4 to 5 AU to avoid the Solar Zodiacal Light (ZL) background at 10 μm instead of four 8-meter ones observing from the Earth vicinity. This allows the mission to be feasible in the near future. The concept, named DARWIN, is under consideration by the European Space Agency for its Horizon 2000 Plus program.

Circumstellar molecular envelopes of AGB and post-AGB objects

Abstract: This review presents the physical and chemical structure of circumstellar envelopes (CSEs) around stars on, and beyond, the asymptotic giant branch (AGB). It focusses on information that is relevant to the interpretation of the molecular line emission from these envelopes. The mass loss rate estimate from CO radio line emission is briefly discussed. It continues with an overview of the morphology and kinematics of AGB and post-AGB CSEs, based on the observational evidence obtained from molecular line emission. Also the small-scale structure of the circumstellar medium is discussed. Finally, a possible scenario for the puzzling, drastic change (at least apparently) from largely spherical AGB-CSEs to the prominent equatorial density enhancements that dominate in CSEs around late post-AGB objects, e.g., planetary nebulae (PNe), is put forward.

Dusty plasmas in interstellar clouds and star forming regions

Abstract: We provide an introduction to self-consistent theoretical models of interstellar dynamics in which dust is treated on a footing equal to that adopted for the description of the gaseous components of the media. Simple background material on the natures of interstellar clouds, dust, and star forming regions is given in the hopes that this review will be accessible to scientists and students interested in a broad range of nonastronomical dusty plasmas as well as to astrophysicists. The main emphasis is on the effects of dust on the damping of waves in clouds, on ambipolar diffusion in star formation, and on the structures of shocks driven into the interstellar clouds by the winds from recently formed stars.

Cosmic Ray Modulation: Effects Of High Speed Solar Wind Streams

Abstract: Based on their source of origin, high speed streams detected in the solar wind plasma have been classified into two classes, coronal hole and solar flare associated streams. Observed heliospheric plasma and field parameters of these streams such as speed, field strength and its variance have been utilized in a systematic manner in order to see their effects in cosmic ray modulation. It is found that flare associated streams are much more effective in modulation than streams from coronal holes. Inspite of the possibility that solar wind structures during two types of streams might be different, the field variance appears to be the most cricial parameter responsible for this difference in their effectiveness in modulation.

Torques and Instabilities in Intermediate Polars

Abstract: The observed spin periods of the primaries in intermediate polars require the presence of torques additional to the standard expectation of material and magnetic torques from an accretion disc. In an extension of earlier work by Wickramasinghe, Wu and Ferrario, from rates of mass transfer deduced from optical and X-ray fluxes, and assuming surface magnetic fields similar to those observed in the most rapidly rotating low mass main sequence stars, it is shown that the magnetohydrodynamic torque between the partially shielded primary and the secondary is sufficient to account for the observed spin periods. It is further found that there is a range of magnetic moments and mass transfer rates in which synchronized rotation of the primary can occur even though it possesses an accretion disc. This may account for the structures deduced for V795 Her and V2051 Oph.

On the origin of solar cycle periodicity

Abstract: Recently, the origin of the solar cycle is considered to be rooted in the dynamics of the solar core (Grandpierre, 1996). The dynamic solar core model requires macroscopic flow and magnetic field as basic inputs. The macroscopic flow cannot be generated by the quasistatic solar structure and it has to reach a larger than critical size (Grandpierre, 1984) in order to survive dissipation. Therefore the flow must be generated by outer agents. The most significant outer agents to the Sun are the planets of the Solar System. These theoretical arguments are supported by observations showing that planetary tides follow a pattern correlating with the solar cycle in the last three and a half centuries (Wood, 1972; Desmoulins, 1995). Therefore the pulsating-ejecting solar core model gives a firm theoretical basis for the interpretation of these largely ignored observations. In this paper a new and simple calculation is presented which enlightens the planetary origin of the eleven-year period and gives a physical basis for a detailed modelling of the dynamo and the solar cycle.

A novel quadruple beam imaging polarimeter and its application to Comet Tanaka-Machholz 1992 X*

Abstract: For observations of solar system objects an imaging polarimeter has been constructed as an auxiliary instrument for a f/15 focal reducer. With this instrument simultaneous measurement of theQ andU Stokes parameters is possible. It contains no moving parts such as a moving λ/2 retarder. The polarizer consists of two Wollaston prisms which are combined to form a single optical element. Their polarization angles differ by 45°. This compound polarizer is placed in the exit pupil of the afocal telescope-collimator system of the focal reducer and splits the exit pupil into halves. In this way four polarized beams with E-vector orientations 0°, 90°, 45°, and 135° emerge from the exit pupil. These are intercepted by the camera lens of the focal reducer system and imaged simultaneously on the CCD detector. With a properly designed field mask at the Cassegrain focus, the four beams are imaged without overlap.

Very small dust particles (vsdp's) in comet c/1996 b2 (hyakutake) *

Abstract: X-ray fluxes observed from comet C/1996 B2 (Hyakutake) are readily explained in terms of scattering by carbonaceous particles with radii of several tens of Angstroms. A few tenths of a megatonne of such particles appear to have been present in the cometary coma on March 26-28, 1996.

High resolution spectroscopy in the far UV: Observations of the interstellar medium by IMAPS on ORFEUS-SPAS

Abstract: The Interstellar Medium Absorption Profile Spectrograph (IMAPS) is an objectivegrating, echelle spectrograph built to observe the spectra of bright, hot stars over the spectral region 950–1150A, below the wavelength coverage of HST. This instrument has a high wavelength resolving power, making it especially well suited for studies of interstellar absorption lines. Following a series of sounding rocket flights in the 1980's, IMAPS flew on its first Shuttle-launched orbital mission in September 1993, as a partner in the ORFEUS-SPAS program sponsored by the US and German Space Agencies, NASA and DARA.

Fresip: A mission to determine the character and frequency of extra-solar planets around solar-like stars

Abstract: FRESIP (FRequency of Earth-Sized Inner Planets) is a mission designed to detect and characterize Earth-sizes planets around solar-like stars The sizes of the planets are determined from the decrease in light from a star that occurs during planetary transits, while the orbital period is determined from the repeatability of the transits Measurements of these parameters can be compared to theories that predict the spacing of planets, their distribution of size with orbital distance, and the variation of these quantities with stellar type and multiplicity Because thousands of stars must be continually monitored to detect the transits, much information on the stars can be obtained on their rotation rates and activity cycles Observations of p-mode oscillations also provide information on their age and composition These goals are accomplished by continuously and simultaneously monitoring 500 solar-like stars for evidence of brightness changes caused by Earth-sized or larger planetary transits To obtain the high precision needed to find planets as small as the Earth and Venus around solar-like stars, a wide field of view Schmidt telescope with an array of CCD detectors at its focal plane must be located outside of the Earth's at mosphere SMM (Solar Maximum Mission) observations of the low-level variability of the Sun (∼1:100,000) on the time scales of a transit (4 to 16 hours), and our laboratory measurements of the photometric precision of charge-coupled devices (1:100,000) show that the detection of planets as small as the Earth is practical The probability for detecting transits is quite favorable for planets in inner orbits If other planetary systems are similar to our own, then approximately 1% of those systems will show transits resulting in the discovery of 50 planetary systems in or near the habitable zone of solar-like stars

Oblique solitary Alfvén modes in relativistic electron-positron plasmas

Abstract: In electron-positron plasmas the charge-to-mass ratio is the same for both species. This leads for different waves to the vanishing of certain coefficients in the dispersion laws and nonlinear evolution equations, and also to the decoupling of some of the plasma modes. In particular, there is a low-frequency mode which exists at all angles of propagation with respect to the static magnetic field, corresponding at parallel propagation to a degenerate case of circularly polarized waves, and at perpendicular propagation to part of the extraordinary mode. The nonlinear evolution of this generalizedX-mode is governed by a Korteweg-de Vries equation, valid at all angles of propagation except strictly parallel propagation, for which a different approach had been given already. The nonlinearity is strongest at perpendicular propagation.

Advances in numerical modeling of astrophysical and space plasmas

Abstract: Plasma science is rich in distinguishable scales ranging from the atomic to the galactic to the meta-galactic, i.e., the mesoscale. Thus plasma science has an important contribution to make in understanding the connection between microscopic and macroscopic phenomena. Plasma is a system composed of a large number of particles which interact primarily, but not exclusively, through the electromagnetic field. The problem of understanding the linkages and couplings in multiscale processes is a frontier problem of modern science involving fields as diverse as plasma phenomena in the laboratory to galactic dynamics.

The formation and evolution of low mass protostars

Abstract: A review is presented of the earliest stages of protostellar evolution. Observations of prestellar cores, which are believed to represent the initial conditions for protostellar collapse, depart significantly from the scale-free density distribution which is usually taken as the starting point for the formation of a low-mass protostar. Pre-stellar cores are observed to have radial density profiles which have flat inner regions, steepening towards their edges. This is seen to qualitatively match the predictions of the Bonnor-Ebert stability criterion for pressure-bounded self-gravitating gas clouds. From these initial conditions, theoretical modelling of cores threaded by magnetic fields predicts that quasi-static evolution by the process of ambipolar diffusion will lead to a significantly different starting point for collapse than the static singular isothermal sphere.

Vortices in strongly magnetized nonuniform electron-positron-ion plasmas

Abstract: The nonlinear mode coupling equations for electrostatic and electromagnetic waves in strongly magnetized nonuniform electron-positron-ion plasmas are derived. It is found that a small fraction of stationary ions (or high-Z charged impurities) can be responsible for the formation of coherent vortices which are forbidden when the ions are absent. Such vortices might significantly affect the transport properties of electron-positron plasmas in external magnetic fields.

A model of light-curve synthesizing for dwarf novae and the analysis of the OY Car observations by application of the inverse-problem method

Abstract: The paper contains a model synthesizing the light curves of novae and novae-like stars, as well as of active close binaries (CB) in the phase of an intensive matter exchange between the components with accretion onto a white dwarf. The model considers the radial and azimuthal temperature distributions in the disk enabling a successful interpretation of asymmetrically deformed light curves characteristic for these systems. The analysis of the observed light curves is performed by using the inverse-problem method (Djurasevic, 1992b) adapted to this model. In the particular case the parameters for the dwarf-novaOY Car are estimated on the basis of the U and B observations (Woodet al., 1989). The synthetic light curves obtained through the inverse-problem solving, as a whole, fit the observations well which indicates that it is possible to estimate the system parameters on the basis of the model proposed here. The obtained results indicate a complex hot-spot structure approximated in the model with two components—a central part and a surrounding spot larger in size. The central hot-spot part (temperature about 10000 K is surrounded asymmetrically by the larger spot lower in temperature (about 7000 K). The radiation of the central hot spot is ‘beamed’ forward by about 20°. The angular size of the hot-spot central part is about 5°, the centre longitude is 322°, whereas for the surrounding spot the size is about 33° and the longitude of the centre about 300°. For the mass ratio of the componentsq=0.102 one finds for the orbit inclination about 83°.8. The analysis shows that the disk radius is about 51% of the corresponding Roche lobe radius. Based on the U and B light curves the quiescent disk-edge temperature is estimated to about 5500 K (U), i.e. 4400 K (B). The disk-radial-temperature profile is much flatter than in the steady-state-approximation case. Beginning from the edge towards the disk centre the temperature slowly increases attaining about 7200 K (U), i.e. 5700 K (B) near the white dwarf. The differences in the solutions for the U and B light curves can be due to deviations in the disk radiation from the black-body approximation assumed in the present model. Expressed in the units of the distance between the component centres [D=1] the disk size is estimated to about 0.304 [D=1], its thickness to 0.014 [D=1], and the white-dwarf radius to about 0.02 [D=1]. The white-dwarf temperature is about 15000 K. The obtained results are in a relatively good agreement with the system parameters estimated earlier (Woodet al., 1989). This indicates that the proposed model of the system and the corresponding inverse-problem method briefly presented here are fully applicable to the analysis of active CB light curves in this evolutionary phase. Though the model given here includes a number of approximations, it enables an independent procedure in the observational-material analysis based on the light-curve synthesis and on the application of the inverse-problem method. Results obtained by applying such an independent method can also serve as a reasonable way in testing the solutions obtained by utilising the earlier approaches.

Microlensing and polarization

Abstract: The discovery of the microlensing effect has found out the new population — dark bodies having mass of the order of 0.1M⊙ which form the halo of our Galaxy. There are two main criteria which identify a light curve as the reveal of the microlensing effect. They are symmetry of the light curve and equal amplification in different colors. We consider polarization of the light beam arising in microlensing effect and find that additional test is possible — polarization of light in the case of a finite star disk. Consequences of polarization are also considered.

Non-adiabatic interactions in excitedC 2 H molecules and their relationship toC 2 formation in comets

Abstract: A unified picture of the photodissociation of theC2H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC2 are formed during the photodissociation of theC2H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC2 radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC2 fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC2H radical. The photochemically active states are the 22∑+, 22II, 32II, and 32∑+, with photodissociation rate constants of 1.0×10−6, 4.0×10−6, 0.7×10−6, and 1.3×10−6s−1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×105 s.

Atomic Processes in Astrophysics

Abstract: We review the atomic processes responsible for astrophysical emission line spectra, paying particular attention to the reliability of the rates used to determine the physical conditions in the emitting gas. We discuss particular cases where often-neglected processes play important roles.

Far-infrared contribution to interstellar extinction from graphite whiskers

Abstract: A mass contribution of graphite whiskers as small as 0.1% to the population of interstellar grains could dramatically change their far-infrared extinction properties. With varying mass fractions of graphite whiskers, and for different size parameters, the infrared extinction could vary from a λ-2 dependence to λ0, consistent with the requirements of some astronomical observations.

Observational manifestations of early mixing in B- and O-type stars

Abstract: The helium and nitrogen enrichment of the atmospheres of early B-type stars during the main sequence (MS) evolutionary phase is re-analysed. It is confirmed that the effect depends on both the aget and the stellar massM. For example, the helium abundanceHe/H increases by 0.04 (60–70% of initial value) for stars withM=8–13M ⊙ and by 0.025 (about 30%) for stars withM=6M ⊙. The nitrogen abundance rises by three times forM=14M ⊙ and by, two times forM=10M ⊙. According to the latest theoretical computations, the observed appearance of CNO-cycled material in surface layers of the stars can be a result of the rotationally induced mixing, in particular, of the turbulent diffusion. Carbon is in deficiency in B stars, but unexpectedly does not show any correlation with the stellar age. However it is shown that the total C+N abundance derived for early B stars conflicts with the theory. Basing on modern data the helium enrichment is first examined in O-type MS stars, as well as in components of binaries. As compared with early B stars, the He abundance for more massive O stars and for components of binaries show a different relation with the relative aget/t MS . Namely during short time betweent/t MS ≈0.5 and 0.7 a sharp jump is observed up toHe/H=0.2 and more. In particular, such a jump is typical for fast rotating O stars (v sini≥200 km s−1),. Therefore the effect of mixing depends on massM, relative aget/t MS , rotational velocityv and duplicity. The mass problem (the discrepancy betweenM ev andM sp ) is also analysed, because some authors consider it as a possible evidence of early mixing, too. It is shown that the accurate data for components of binaries lead to the conclusion that the discrepancy is less than 30%. Such a difference can be removed at the expense of theM ev lowering, if the displacement of evolutionary tracks, owing to the rotationally induced mixing is taken into consideration.